It is a known phenomenon that the modules of certain photovoltaic generators, particularly so-called thin film modules, achieve their maximum lifetime only if they are completely on a negative or a positive electric potential with regard to earth potential. A DC voltage source providing an offset voltage may be used to ensure such an electric potential of the modules of a photovoltaic generator with regard to earth potential.
EP 2 136 449 B1 discloses a circuitry arrangement for a solar power plant. This circuitry arrangement includes an inverter, a transformer providing a galvanic separation of all electric lines carrying alternating current between an inverter and a grid terminal, and an offset path leading from one of the electric lines at the primary side of the transformer to earth potential. A DC voltage source is arranged in the offset path and provides an offset voltage for adjusting an electric potential of a photovoltaic generator connected to the DC side of the inverter with regard to earth potential. The transformer provides the galvanic isolation between several transformerless inverters connected in parallel and the grid terminal. If the transformer comprises a center point connection at its primary side, the offset path is connected to the center point. If the transformer comprises a delta configuration at its primary side, the offset path is provided between one of the phase lines and earth potential. The inverters connected in parallel each feed electric power from one photovoltaic generator into one of three phase lines to the transformer. In transformerless inverters, a potential shift by a certain DC component at the AC or output side also results in a potential shift of photovoltaic generators connected to the DC or input side and vice versa. The offset voltage of the DC voltage source thus has the effect that the photovoltaic generators connected to the inverters are completely at positive electric potential. Additionally, a current surveying or monitoring device of the DC voltage source may be used for identifying earth faults. In case of an isolation or earth fault, an increased current via earth potential flows through the offset path. In case of an isolation or earth fault, a contactor is triggered that interrupts the current connection between the inverter and the transformer or between the grid terminal and the transformer.
WO 2010//051812 A1 discloses a further circuitry arrangement for a solar power plant. Here, single- or three-phase transformerless inverters are connected in parallel and each feed electric power from one photovoltaic generator via a common transformer providing a galvanic separation into an AC power grid. A DC voltage source for an offset voltage is connected to the center point of the primary side of the transformer, independently of whether this center point is connected to the individual inverters or not. In this known circuitry arrangement, the height of the offset voltage which is provided by the DC voltage source is adjusted to the present output voltage of the photovoltaic generators. Thus, the electric potential of the photovoltaic generators is always shifted by an offset voltage which is just sufficient to render the electric potential of the photovoltaic generators completely positive or negative.
According to WO 2010/051812 A1, only very small currents flow through the DC voltage source for the offset voltage so that only very little electric power is dissipated due to the offset voltage. Actually, the power is estimated as having an order of 1 W only. This may be correct with regard to the current generated by the offset voltage as long as there is no fault current via earth. Such a fault current may, however, readily have an order of 1 A. With a typical offset voltage of several 100 V, this means several 100 W of power. Additionally, potential variations which occur in the operation of usual inverters at their DC side, together with a high electric capacitance of the modules of photovoltaic generators with regard to earth potential, result in high capacitive leakage currents which flow back through the DC voltage source in the offset path. The capacitive leakage currents have a typical order of some 100 mA up to several amperes and strongly depend on the weather. The electric capacitance of modules of photovoltaic generators with regard to earth potential, for example, strongly varies with air moisture and particularly due to rainfall onto the modules. A comparatively high alternating current component of the current through the offset path towards earth potential is thus no indication of an earth or isolation fault, but considerably stresses the DC voltage source arranged in the offset path. On the other hand, even a comparatively low direct current through the offset path is an indication of a dangerous fault and has thus to be switched off immediately or at least to be reduced to a safe value.
Particularly, such a fault current has to be immediately switched off or at least reduced to a safe value for security reasons if the solar power plant shall be operated further for feeding further electric power into a connected AC power grid. Here, it has to be taken into consideration that the current through the offset path—particularly in case of a fault—is a mixture of alternating current and direct current components with a comparatively high direct current component. A circuit breaker sensitive to mixed currents, particularly an RCD type B, is not suited for securely switching off such a fault current as this device only switches off a mixed current having a small direct current component at maximum. With a higher direct current component, an electric arc occurs between the contacts of the device via on which the fault current flows. Thus, it is not possible to protect the known circuitry arrangements for solar power plants by a usual circuit breaker in the offset path which switches off both any fault currents occurring and the earth reference via the offset path so that an isolation or earth fault only results in different earth reference, but does not require that the solar power plant is switched off instantaneously.
A further circuitry arrangement for adjusting the electric potential of photovoltaic generators of a solar power plant is known from WO 2010/078669 A1. Here, an offset path consists of a potential shifting device made as a DC voltage source for generating a DC voltage between earth potential and an output of an inverter. The DC voltage source consists of a transformer, a rectifier and a voltage regulator, a capacitor being connected in parallel to its two outputs. At its primary side, the transformer is connected to the outputs of the inverter. On its secondary side, the transformer is connected to the rectifier.
EP 2 086 020 A2 discloses a photovoltaic power plant with potential shift. Here an offset path extends from earth potential to one of two inputs of an inverter. A DC voltage source, a current sensor and a circuit breaker are arranged in the offset path. The circuit breaker is opened if the current registered by the current sensor exceeds a threshold value. The current flowing through the DC voltage source and measured by the current sensor is a mixed current of alternating current and direct current components. Thus, there is basically the same problem as with the circuitry arrangement known from WO 2010/051812 A1: A comparatively high alternating current component of the current through the offset path towards earth potential is not yet an indication of an earth or isolation fault although it considerably stresses the direct voltage source arranged in the offset path.
DE 1 255 783 discloses a earth fault protection device for galvanically connected direct and alternating voltage systems. The earth fault protection device comprises two paths connected in parallel. In the first parallel path, a resistor for current limitation, a first direct current relay for indicating an earth fault and a DC source are connected in series. Capacitors for sieving out alternating current are connected in parallel to the first direct current relay and the DC source. In the second parallel path, a capacitor blocks direct current, and a resistor limits the current. Further, a second direct current relay is arranged in such a way that it travels if an alternating current flows through the second parallel path. The first and second direct current relays thus indicate a flowing direct current on the one hand and a flowing alternating current towards earth on the other hand.
There still is a need for a circuitry arrangement for a solar power plant solving the problems which are connected with leakage currents flowing via the offset path.